A polymer electrolyte fuel cell (hereinafter referred to as a “PEFC”) generates electricity and heat at the same time by causing a hydrogen-containing fuel gas obtained by reforming a material gas, such as a city gas, and an oxygen-containing oxidizing gas, such as air, to electrochemically react with each other. A unit cell (cell) of the PEFC includes an MEA (Membrane Electrode Assembly; polymer electrolyte layer-electrode stack body), gaskets, and electrically-conductive plate-shaped separators. The MEA is constituted by a polymer electrolyte layer and a pair of gas diffusion electrodes. A groove-like gas channel through which the fuel gas or the oxidizing gas (each of these gases is called a reactant gas) flows is formed on a main surface of the separator which surface contacts the gas diffusion electrode. A groove-like cooling medium channel through which a cooling medium for recovering generated heat and cooling down an inside of the cell is formed on a main surface opposite to the main surface contacting the gas diffusion electrode. A pair of separators sandwich the MEA, around a peripheral portion of which the gaskets are disposed. Thus, the cell is formed. A cell stack is formed by stacking a plurality of cells, sandwiching the stacked cells by end plates from both sides, and fastening the end plates and the cells using fastening members.
In the case of using such PEFC as a domestic fuel cell, the PEFC recovers the generated heat and utilizes the heat for a domestic hot-water supply. Thus, energy use efficiency is improved. Conventionally, in the domestic fuel cell, the temperature of the hot water used in the hot-water supply is around 60° C. However, in the case of considering the reduction in size of a hot water tank and the application to floor heating, the hot water of higher temperature is required. In order to generate the hot water of such high temperature, the temperature of the cooling medium at an exit of the cell stack needs to be increased.
Moreover, in the case of using the PEFC as a car fuel cell, the car fuel cell needs to generate higher output than the domestic fuel cell, so that it generates a large amount of heat. Therefore, in order to efficiently cool down the cell stack within a limited mount space, the temperature difference between the cooling medium and outside air needs to be large. On this account, in the car fuel cell, the temperature of the cooling medium at the exit of the cell stack needs to be as high as possible.
In order to increase the temperature of the cooling medium at the exit of the fuel cell stack, the operating temperature of the cell stack needs to be increased. However, if the operating temperature is increased, a relative humidity around the MEA decreases, polymer electrolytes contained in the polymer electrolyte membrane and the catalyst layer dry, ion conductivity deteriorates, and power generation efficiency deteriorates. Moreover, in order to increase the relative humidity around the MEA, it is effective to increase a humidification temperature of the reactant gas. However, in order to do this, the performance of a humidifier needs to be increased, and this leads to the increase in cost and size of the fuel cell system.
To solve the above problems, known is a fuel cell in which: each of a first cell assembly and a second cell assembly is constituted by a plurality of unit cells; and in order to supply and/or circulate the reactant gas and the cooling medium to or in the first cell assembly and the second cell assembly, a reactant gas channel and a cooling medium channel are formed so as to be communicated with each other in series in the first cell assembly and the second cell assembly (see PTL 1 for example). In accordance with the fuel cell disclosed in PTL 1, the cooling medium which has cooled down the first cell assembly is supplied to the second cell assembly. Therefore, the second cell assembly is basically higher in temperature than the first cell assembly. Thus, the cooling medium discharged from the second cell assembly can be easily increased in temperature.